Breton smocks were designed , under a thin fabric layer, to prevent wind from getting through clothes and to keep warmth inside. It is why traditional breton smocks were made out of a very tight woven cotton fabric (at first the same as the fabric used to make sails).

Sailcloth breton smock
made out of a strong canvas cotton fabric.
Two inside pockets
button for collar.
it is the breton fishermen smock
by mousqueton